The present invention relates generally to measurement of acoustic resonances in liquids and, more particularly, to the identification of certain properties of liquids through the comparison of their corresponding acoustic resonant frequencies.
Various state and federal laws require that the octane ratings posted on gasoline pumps at gas stations are within certain limits of accuracy. Octane rating is a number indicating the degree of knocking of a fuel mixture under standard test conditions. To prevent the fuel-wasting, potentially damaging engine knock at all engine speeds and loads, gasoline must have high antiknock quality (octane number) throughout its entire distillation range.
Early in the study of engine knocking, it was recognized that the chemical structure of fuel hydrocarbons largely determines their tendency to cause knock, and that straight-chain paraffins are more prone to knocking than branched-chain paraffins, olefins, or cyclic hydrocarbons. Soon after the discovery of antiknock additive agents, it became evident that a yardstick was needed for measuring the antiknock quality of motor fuels. In 1926, Ethyl Corporation developed the octane scale, which has become the worldwide standard for that purpose. For the zero of the scale, a straight-chain hydrocarbon, n-heptane, is selected since it burns with considerable knock. For 100, a non-knocking branched-chain hydrocarbon 2,2,4-trimethylpentane (often incorrectly referred to as isooctane) is chosen. By blending these two hydrocarbons in varying percentages, a primary reference fuel can be produced to match the knock resistance of any gasoline sample. Octane number is defined as the percentage of isooclane required in a blend with n-heptane to match the knocking behavior of the gasoline being tested. Thus, if a blend of 87% 2,2,4-trimethylpentane and 13% n-heptane is required to match the knock resistance of a particular gasoline sample when both are run in a test engine under specified conditions, the sample is said to have an octane number of 87.
The CFR (Cooperative Fuel Research) knock-test engine has been adopted as the standard for determining octane number. Basically, it is a single-cylinder, four-stroke engine in which the compression ratio can be varied at will. Auxiliary equipment includes means for detecting pressure impulses from detonation, an electronic amplifier, and a meter to record knock intensity. To determine a fuel's antiknock quality, the CFR engine is operated on the fuel under a standard set of conditions, and its compression ratio is adjusted to give a standard level of knock intensity. The method is cumbersome and time-consuming, and the equipment is expensive. For frequent monitoring of gasoline octane rating at gas stations and at distillation plants, a simpler method is needed.
It has been known for several decades that it is possible to set-up acoustic interference patterns inside a liquid if the liquid is contained within two parallel-plate acoustic transducers. Generally, one uses thin quartz disks for transducers. One transducer is excited by a swept sine signal while the other transducer picks up the signal that results from interferences within the liquid at certain fixed frequencies which depend on the separation between the two transducers and the speed of sound in the liquid. The interferences are detected as resonances and can be easily observed using commercially available electronics. This conventional resonator technique as described requires very thin and fragile quartz discs which must be kept in contact with the liquid inside a chamber.
Accordingly, an object of the present invention is to provide an apparatus for comparing corresponding acoustic resonant frequencies of liquids in identical receptacles.
Another object of the invention is to provide an apparatus for determining the resonant frequencies of liquids without contacting the liquid.
Yet another object of the present invention is to determine the identity of liquids and other characteristics thereof which affect the speed of sound therein from a comparison of their corresponding acoustic resonances.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.